This invention relates to the recovery and purification of acrylonitrile made by catalytic ammoxidation of propylene.
In commercial processes for preparation of acrylonitrile from propylene, ammonia, and oxygen (air), the reactor effluent contains, in addition to the desired acrylonitrile product, considerable amounts of by-product hydrogen cyanide, acetonitrile, and other impurities such as succinonitrile and other nitriles. The exact composition of the effluent and the by-products and impurities it contains may vary considerably depending on the ammoxidation reaction conditions and catalyst.
Processes for treating reactor effluents of the type described to separate and recover acrylonitrile product and desired by-products such as hydrogen cyanide and acetonitrile are known. For example, see U.S. Pat. Nos. 3,399,120; 3,433,822; 3,936,360; 4,059,492; 4,166,008; and 4,404,064, the disclosures of which are incorporated herein by reference. Typically, these processes include introducing the reactor effluent into a quench chamber where it is contacted with water (usually containing sulfuric acid to neutralize excess ammonia from the reaction) to cool the effluent and remove some contaminates such as polymers produced in the reactor. Cooled effluent gases from the quench flow to an absorber column where they are contacted with water. The liquid stream from the bottom of the absorber column contains most of the nitriles produced in the reaction and impurities and is sent to an extractive distillation column. The major portion of the acrylonitrile from the extractive distillation column is obtained in the overhead (distillate) from the column while water and impurities constitute the bottom stream from the column. In accordance with practices of the art, the bottom stream is frequently fed to a secondary distillation or stripper column to separate acetonitrile and water in an overhead stream while the secondary column bottoms containing water and various impurities are recycled to the quench column. It was apparently believed that the impurities in the recycle stream were acceptable in the quench system (see, for example, U.S. Pat. No. 3,960,360). The large quantity of quench liquid required by the quench column and waste management considerations make the appropriate use of recycle water an important process consideration. Accordingly, improvements in recycle practices are sought by those skilled in the art.